Various ideas have been devised in order to increase a transmission capacity in the radio communication system (hereinafter, referred to as a “system capacity” in some cases) of the related art. For example, in 3rd generation partnership project long term evolution (3GPP LTE), a technique for increasing the system capacity by utilizing a “small cell” other than a “micro cell” has been discussed. Here, the “cell” is a term which indicates a range covered by a radio base station so that a radio terminal transmits and receives a radio signal, but since the concept of the radio base station and the concept of the cell almost correspond to each other, the “cell” and the “radio base station” may be properly replaced with each other in the following description. Then, the “micro cell” represents a cell of a base station capable of transmitting a signal at a high transmission power level, that is, a cell of a base station having a large coverage area. In addition, the “small cell” is a cell of a base station transmitting a signal at a low transmission power level, that is, a cell of a base station having a small coverage area.
In the 3GPP LTE, as a configuration of a mobile communication system, for example, a configuration of including a plurality of small cells in the micro cell has been considered. In addition, a technique of concurrently connecting the micro cell and the small cell to the mobile terminal has been considered. Additionally, a technique of concurrently connecting two small cells, which are different from each other, to the mobile terminal has been considered. In this manner, the communication executed by being connected two different cells to the mobile terminal is referred to as dual connectivity in some cases. In addition, the base station, with the dual connectivity, is directly connected to a device of a superordinate layer in some cases. Note that there is no limit to this configuration. Generally, the dual connectivity is configured so that the terminal is connected to a plurality of base stations and thus concurrently communicates with each of the base sections, thereby transmitting and receiving different pieces of information concurrently with each of the base stations. In the description, the dual connectivity will be described, but similar discussion may be applied to a case of multiple-connectivity, that is, more than ternary connectivity. For this reason, the dual connectivity in the following description may be regarded as the concept including the multiple-connectivity, or may be replaced with the multiple-connectivity.
In a case where the mobile terminal concurrently is connected to the micro cell and the small cell, for example, a control plane for transmitting L3 control information is connected to the micro cell, which performs the setting of a transmission line for transmitting data and the control of a handover. In addition, a data plane for transmitting and receiving data is connected to the small cell. Here, the control plane is also referred to as a C-plane or a signaling radio bearer (SRB). Further, the data plane is also referred to as a user plane, a U-plane, or a data radio bearer (DRB).
In addition, in a case where the mobile terminal is concurrently connected to two different small cells, for example, a configuration in which the control plane is connected to one small cell and the data plane is connected to the other small cell, or a configuration in which the data plane is connected to both of the base stations may be considered. As described above, generally, the dual connectivity is configured so that the terminal is connected to a plurality of base stations and thus concurrently communicates with each of the base sections, thereby transmitting and receiving different pieces of information concurrently with each of the base stations.
As such, in the dual connectivity, a radio base station to which the control plane is connected is referred to as a primary radio base station (cell) in some cases. In addition, a radio base station (cell) to which the data plane is connected and which cooperates with the primary radio base station (cell) to perform data communication is referred to as the secondary radio base station in some cases. In addition, each of them is referred to as an anchor radio base station (cell), or an assisting radio base station (cell) in some cases. Furthermore, each of them is referred to as a master radio base station (cell) or a slave radio base station (cell) in some cases. Here, in a case of the dual connectivity, when a radio communication terminal is capable of receiving data from two radio base stations, both of the primary and secondary radio base stations may be configured to output data to the radio communication terminal so as to improve communication properties. Note that names of the radio base stations are not limited to those in this description. Generally, similar to an LTE communication system of the related art, as long as the radio base station which performs communication by connecting both of the control plane and the data plane is a main base station, it is possible to use various names within the scope of the disclosures.
As a configuration of the dual connectivity, various configurations have been proposed according to layers separating the data plane.
For example, there is a configuration of separating the data plane in a previous stage of a packet data convergence protocol (PDCP) layer. In addition, for example, there is a configuration of separating the data plane between the PDCP layer and a radio link control (RLC) layer. Furthermore, for example, there is a configuration of separating the data plane between the RLC layer and a medium access control (MAC) layer. However, a configuration of separating in the layer may be employed without limiting the above described configurations. For example, it is possible to employ a configuration in which the primary base station performs a function of a portion of the PDCP layer, and the secondary base station performs the remaining functions of the PDCP layer. In addition, the same is also true for the RLC layer and the MAC layer.
When employing the configuration of the dual connectivity, the primary radio base station and the secondary radio base station are connected to each other via a wired link or a wireless link. Then, after separating the data plane, data is transmitted to the secondary radio base station via a link connecting the primary radio base station and the secondary radio base station.
An example of the related art includes Japanese Laid-open Patent Publication No. 2011-250211.